Robin Williams was not only a father and a husband, but also a deeply devoted philanthropist. In particular he had a special relationship to our men and women serving overseas as ABC's Martha Raddatz reports. Reporter: There was an instant connection between robin Williams and our troops. Never phoney, never the privileged celebrity. Just a comedian who helped them escape the cruelty of war or the aftermath of injuries, if just for a moment. He was the comedian who during USO tours would sneak away after his big performance to visit the troops who weren't able to attend. The dog handlers, the dining hall workers, the soldiers keeping guard. It was in Kuwait in 2007, entertaining the troops, when robin Williams had an unforgettable moment himself. Oh-oh! Reporter: Retreat sounded. Signaling the lowering of the flag. And everyone turned to face it. Williams, not quite knowing what was happening, just paused. But there was that pride, that respect, and then so quickly afterwards, the humor. I'm not going to forget that! I've never had an entire audience just go, forget you! Reporter: Our troops knew he was one of the special ones. A man who truly cared and showed it. I'm Martha Raddatz for "Nightline" in Washington. Tonight the secretary of defense called Williams a true friend and supporter of our troops.

This transcript has been automatically generated and may not be 100% accurate.

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Williams, who has been open about his addictions, has been sober for years.

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